1. Field of the Invention
The present invention relates to a method of manufacturing tyres for vehicle wheels.
The invention also relates to an apparatus for manufacturing tyres for vehicle wheels.
2. Description of the Related Art
A tyre for vehicle wheels generally comprises a carcass structure comprising at least one carcass ply having respectively opposite end flaps looped back around annular anchoring structures, usually each formed of a substantially circumferential inner insert onto which at least one filling insert is applied, at a radially external position.
Associated with the carcass structure is a belt structure comprising one or more belt layers placed in radial superposed relationship with respect to each other and the carcass ply and having textile or metallic reinforcing cords with a crossed orientation and/or substantially parallel to the circumferential extension direction of the tyre. At a radially external position to the belt structure a tread band is applied which is made of elastomer material like other semifinished products constituting the tyre.
It is to be herein pointed out, to the aims of the present description, that by the term “elastomer material” it is intended a composition comprising at least one elastomer polymer and at least one reinforcing filler. Preferably, this composition further comprises additives such as cross-linking agents and/or plasticizers. Due to the presence of cross-linking agents, this material can be cross-linked through heating so as to form the final article of manufacture.
Also applied to the side surfaces of the carcass structure, each extending from one of the side edges of the tread band until close to the respective annular anchoring structure to the beads, are respective sidewalls of elastomer material that, depending on the different embodiments, can have respective radially external end edges that are superposed on the side edges of the tread band to form a design scheme of the type usually called “overlying sidewalls”, or interposed between the carcass structure and the side edges of the tread band itself, in accordance with a design scheme of the type referred to as “underlying sidewalls”.
In most of the known processes for tyre manufacture, the carcass structure and belt structure are provided to be made separately of each other in respective work stations, to be mutually assembled at a later time.
More particularly, manufacture of the carcass structure first contemplates deposition of the carcass ply or plies on a first drum usually identified as “building drum” to form a cylindrical sleeve. The annular anchoring structures to the beads are fitted or formed on the opposite end flaps of the carcass ply or plies that in turn are turned up around the annular structures themselves so as to enclose them in a sort of loop.
Simultaneously, on a second drum or auxiliary drum, an outer sleeve is made which comprises the belt layers laid down in radially superposed relationship with each other (and optionally the tread band applied to the belt layers at a radially external position). The outer sleeve is then picked up from the auxiliary drum to be coupled with the carcass sleeve. For the purpose, the outer sleeve is disposed in coaxial relation around the carcass sleeve, and afterwards the carcass ply or plies are shaped into a toroidal conformation by axially moving the beads close to each other and simultaneously admitting fluid under pressure into the carcass sleeve, so as to determine application of the belt ring (and optionally of the tread band) to the carcass structure of the tyre at a radially external position thereof. Assembling of the carcass sleeve with the outer sleeve can be carried out on the same drum as used for building the carcass sleeve, in which case reference is made to a “unistage building process”. A building process of this type is described in document U.S. Pat. No. 3,990,931, for example.
In manufacturing some particular tyre types, such as in the case of tyres for lorries and the like, application of so-called “underbelt inserts” interposed between the carcass structure and belt structure is provided.
Here and in the following of the specification and claims, by the term “underbelt insert” it is intended a substantially circumferential insert preferably of elastomer material that is interposed between the carcass structure and belt structure of the tyre; these inserts are preferably present in pairs and each insert is positioned close to each tyre shoulder.
For manufacture of the underbelt inserts, one or more lengths of appropriate cross-section conformation, cut from a continuous strip of extruded elastomer material are circumferentially applied to the cylindrical sleeve formed on the building drum. During the shaping step of the carcass structure for coupling with the belt structure, the underbelt inserts are submitted to a circumferential stretching action imposed by the expansion of the carcass structure, and they modify their orientation in such a manner that, at the end of the shaping step, they form a substantially continuous surface with a radially external portion of the carcass ply, which surface is adapted to evenly bear against the radially internal surface of the belt structure.
Also known are building processes of the so-called “two-stage” type in which, as described in document EP 0 613 757 for example, assembling between the carcass structure and belt structure is carried out on a so-called shaping drum onto which the carcass sleeve picked up from the building drum and, subsequently, the outer sleeve picked up from the auxiliary drum are transferred.
In document WO 01/08874, in the name of the same Applicant, it is described a shaping drum to be used in a two-stage building process and set to facilitate application of the underbelt inserts to the carcass sleeve, before carrying out the shaping step.
Document U.S. Pat. No. 4,392,899, also in the name of the same Applicant, discloses an auxiliary drum such conceived as to obtain a reduction in the required time for assembling the belt structure and tread band on the auxiliary drum.
In accordance with the present invention, the Applicant however realised that important improvements to the production methods of the known art can be made both in terms of productivity and in terms of quality of the obtained product.
In this connection, it could be noticed that for the operations to be executed at a primary building and/or shaping drum for the purpose of making the carcass structure, a higher cycle-time is required as compared with the time required for the operations carried out on the auxiliary drum to make the belt structure and possibly the tread band. In more detail, the Applicant has ascertained that one of the causes leading to an increase in the overall cycle time, i.e. the time required for completing the building cycle of the tyre, as well as leading to geometrical faults of the tyre structure that can adversely affect the quality of the finished product, resides in the necessity to make the underbelt inserts on the carcass structure.